Various methods of converting, part or all of, the C—H bonds in a hydrocarbon compound are known. Examples include a method of using cobalt trifluoride, a method of conducting fluorination reaction by using as a fluorine source hydrogen fluoride generated in electrolysis in an electrolytic bath (generally referred to as electrolytic fluorination) and a method of conducting fluorination directly in the liquid phase by using fluorine gas (generally referred to as direct fluorination) as disclosed in SANDFORD, G. Perfluoroalkanes. Tetrahedron:Tetrahedron: 0040-4020. 2003, vol. 59, p. 437-454. and in CHAMBERS, R. D., et al. Elemental fluorine. Part 11 [1]. Fluorination of modified ethers and polyethers. Journal of Fluorine Chem. 2000, vol. 101, p. 97-105.
The fluorination method using cobalt trifluoride and the electrolytic fluorination method may cause isomerization, cleavage of the main chain and/or recombination reactions thus reducing the yield in the desired end product.
On the other hand an issue encountered in the direct fluorination with fluorine is the increasing difficulty in the hydrogen substitution with increasing fluorine content in the compound. With increasing numbers of neighbouring fluorine atoms C—H bonds become more electron deficient and consequently hydrogen-abstraction by electrophilic fluorine atoms becomes more difficult. To prevent carbon-carbon bond cleavage during a fluorination reaction the heat generated in the reaction has to be rapidly dissipated. Accordingly, fluorine is generally provided under dilute conditions, typically diluted with an inert gas. Thus, in the initial stages of the fluorination reaction dilute fluorine is used and the reaction system is cooled. However, as the fluorination reaction progresses and fluorination becomes increasingly difficult the concentration of fluorine has to be increased and the temperature generally raised. The use of UV light to activate the final stages of the fluorination reaction has been disclosed for instance in EP 344935 A (DU PONT DE NEMOURS AND CO.) Jun. 12, 1989 and in U.S. Pat. No. 4,960,951 (DU PONT DE NEMOURS AND CO.) Feb. 10, 1990 both directed to the preparation of perfluorinated polyethers with elemental fluorine.
Addition of aromatic compounds, such as benzene, hexafluorobenzene or toluene, to promote the final stages of the perfluorination reaction of hydrogenated ethers and polyethers with molecular fluorine has been disclosed for instance in U.S. Pat. No. 5,093,432 (EXFLUOR RESEARCH CORPORATION) Mar. 3, 1992 which relates to a process for the liquid phase fluorination of ethers and polyethers with fluorine in a perhalogenated liquid medium. The use of trichloroethylene is mentioned as a cosolvent to improve the solubility of the hydrogenated starting material in the perhalogenated liquid medium.
However, the above described processes have the drawback that, to obtain a fully fluorinated product, a large excess of fluorine over the stoichiometrically required quantity, is needed. The need to provide a large excess of fluorine over the stoichiometrically required quantity makes it difficult to control the degree of hydrogen substitution when only partial fluorination of the starting compound is required.
There is thus still a need in the art for a process for the fluorination, full or partial, of hydrocarbon compounds, in particular alkanes and ethers, comprising a fluorination step that may be carried out under mild conditions and providing high yields. Additionally it would be highly advantageous to have a process wherein the substitution of C—H bonds with C—F bonds proceeds in a stoichiometric manner and with complete conversion of the fluorine so that the degree of fluorine substitution can be controlled by controlling the amount of fluorine fed to the reaction system.